Compact magnetic probes for use in magnetic resonance imaging (MRI) and magnetic resonance therapy (MRT) systems are known in the art.
U.S. Pat. No. 5,900,793 to Katznelson et al., filed Jul. 23, 1997, entitled "PERMANENT MAGNET ASSEMBLIES FOR USE IN MEDICAL APPLICATIONS" assigned to the common assignee of the present application, incorporated herein by reference, discloses compact yokeless magnet assemblies comprising coaxial annular permanent magnets.
U.S. patent application Ser. No. 09/161,336 to Zuk et al., filed Sep. 25, 1998, entitled "MAGNETIC APPARATUS FOR MRI" and assigned to the common assignee of the present application, incorporated herein by reference, discloses, inter alia, yokeless magnet assemblies comprising coaxial annular permanent magnets and having external gradient coils.
While such compact yokeless permanent magnet assemblies may have many advantages, it may be desirable for some MRI/MRT applications to increase the strength of the main magnetic field of the magnet assemblies. This may require the design of yoked annular permanent magnet assemblies.
Methods for designing yoked and non-yoked permanent magnets are known in the art.
Current yoked magnet design methods, which are also applied to the design of MRI/MRT yoked magnets, are typically based on the initial design of a yoked field generator which typically includes magnetic material and a pole-piece made of a ferromagnetic material such as soft iron or the like. This field generator is pre-designed in a first design stage to achieve a specified magnetic field strength within a specified imaging volume, also known in the art as the field of view (FOV) of the MRI/MRT device. Theoretically, if the surface of the designed field generator was of an infinite size, the magnetic field within the FOV would be highly homogeneous and no further design stages would be required. However, due to practical considerations such as, inter alia, magnet size constraints dictated by the specific application and the cost of constructing large magnets and mechanically supporting them, the size of the field generator must be finite. This practical size limitation of the field generator introduces inhomogeneity in the strength of the magnetic field within the FOV due to edge effects. Therefore, a second design stage is necessary for correcting the inhomogeneities introduced by the finite size of the field generator. This separate second design stage includes the design of additional magnetic and/or ferromagnetic structures for canceling high-order harmonics of the magnetic field generated by the field generator.
U.S. Pat. No. 5,495,222 to Abele et al. discloses a hybrid magnetic structure for generating a highly uniform magnetic field including a yoked primary magnetic system, a secondary magnetic structure comprising a number of prismatic permanent magnetic structures and pole-pieces and a field distortion compensating means such as primary and/or secondary filter structures for compensating magnetic field distortions in the magnetic field.
U.S. Pat. No. 5,475,355 to Abele et al. discloses a method and apparatus for compensation of field distortion in a magnetic structure using spatial filter. The method is based on designing an open magnetic structure having two spaced apart ferromagnetic elements defining a cavity including an imaging region, means for producing a magnetic field having undesirable harmonics in the imaging region and means for reducing undesirable harmonics in the region. The design method taught by Abele et al. involves designing the open magnetic structure and reducing the distortion within the imaging region by successively eliminating orders of harmonics by inserting a predetermined amount of magnetized material between the ferromagnetic elements and a first and second plurality of additional ferromagnetic elements spaced adjacent thereof.
The method of Abele et al. has the disadvantage that it must use sequential steps for sequentially eliminating orders of harmonics. Therefore, since each of these sequential steps is aimed only at eliminating a specific harmonic, this method does not provide optimization of the contribution of each of the sequential steps to the overall desired magnetic field. As a result, the final design of the magnetic structure while having eliminated specific harmonics may not be optimal with regard to the compactness of dimensions of the designed open magnetic structure.
Thus, a general disadvantage of the prior art design methods is that the optimization performed during the design of the additional "correcting" magnetic structures in the second design stage is directed mainly towards canceling the high order harmonics of the magnetic field of the field generator in order to achieve a specified homogeneity within the FOV. Therefore, the second design stage does not provide an optimization of the contribution of the additional magnetic structures to the main field strength within the FOV. Therefore, use of prior art design methods for designing yoked magnet assemblies for MRI/MRT or for other applications may result in magnets which while achieving magnetic field homogeneity within the specified value, are not necessarily optimally compact. This may have further disadvantages such as the need to use relatively large and costly additional magnetic structures for correcting high order harmonics without providing the ability to fully utilize their magnetic field for increasing the achievable strength of the main magnetic field of the designed magnet or MRI/MRT magnetic assembly.
An additional common problem encountered with permanent magnet assemblies is the presence of magnetic field inhomogeneities which are caused by local defects in the material from which the permanent magnets are manufactured. Such defects may result in "hot spots" which are regions of local increase in the magnetic field strength. These inhomogeneities degrade the MRI image quality. Thus, it is desirable to reduce such inhomogeneities.